1. Field of the Invention
The invention relates to a compression device for an internal combustion engine, and more particularly to a centrifugal compressor for a vehicular turbocharger.
2. Description of the Related Art
In recent years, as the power of the vehicular engines increases, much higher requirements have been imposed on the pressure ratio of centrifugal compressors in turbochargers than ever before. However, restricted by the size, the conventional turbocharger can only employ a single centrifugal impeller to process the air. When the compressor operates at a working condition of high rotational speed, strategies of acquiring a higher pressure ratio by improving the rotational speed is restricted by the intensity of the impeller materials; whereas when the compressor operates at a working condition of low rotational speed, the compressor has a low responsibility and a low pressure ratio. Thus, to improve the pressure in the inlet of the motor by improving the pressure ratio of the conventional centrifugal compressor is largely limited.
As shown in FIG. 1, a typical centrifugal compressor for a turbocharger includes: a housing 1, a centrifugal impeller 2, and an air diffusing channel 9. In an ordinary working condition, the centrifugal impeller 2 rotates at a high rotational speed under the drive of a turbine shaft 13, so that the fresh air is drawn into the compressor via an inlet 4 and compressed by a centrifugal force. The compressed air flows into the air diffusing channel 9, and part of a kinetic energy is converted into pressure energy. At the same time, the air flows into an air flow channel 14 via the air diffusing channel 9, and consequently reaches the internal combustion chamber via an outlet of the compressor. In the whole process, only the centrifugal impeller 2 does work on the air, which is very limited. Furthermore, because the size limitation, it is difficult to develop a multiple compression technology. Thus, a compression device, which not only has a size more or less the same as the conventional, but also effectively improves the pressure ratio and a responsibility at low speed, is desired.